What trigger?  Some 2 stage triggers have been known to do that with psa lower and bcgs....

Pull the FCG, and clean the parts.   Hence may have a blow primer hiding under the FCG.  
When you have the disco and trigger out, take a very good look at the tails of them for signs that the hammer tail is peening the hell out of them (very bad thing, and is caused by the not enough mass and spring tension keeping the bolt forward at ignition.  

Also, the 9mm rig loves to snap FCG pins. For this reason, you really want to go with pins that have side suports that are bracked together.  Hence if the pin does snap, the side suports will keep the pin still working. And yes for a 9mm rig, prefer that suported pins that have both bolt on all four ends of the pins.
http://www.operatorchan.org/st/src/135224547687.jpg


When you reinstall the cleaned and lubed FCG, large coil of the disco spring goes into the tail of the trigger, and get the hammer spring correct as well.





Now banking that you have a ramped bolt, the first thing you need to do is hold the trigger back on the empty rifle, slowly pull back on the charging handle all the way back, walk it back forward, then release the trigger to confirm that the ramped carrier actual cocked the hammer back enough for the disco to grab it.  If you get the normal disco release to trigger on trigger release, then second pull of the trigger sends the hammer to strike the FP, then all is good.  If you don't, then bank that you are running a top of contact pad L cut hammer, and will need to change it out with a modified (auto sear tab ground off) M-16 type hammer.

This is not the type of hammer to use with a ramped 9mm carrier,



This is the type to use with a ramped 9mm bolt.


None ramped 9mm bolt left, ramped bolt on the right,



Now shotgun the upper open and without touching the trigger cocked the hammer back until you can get the rear hammer sear as close to the disco sear.  Again, without the trigger touched, the back hammer sear should only clear the disco sear .001"~.003" as it passes by the disco sear (about the thickness of a sheet of paper).  If the free gap is much larger, then the disco may need to be re-tuned.
https://www.ar15.com/forums/t_3_66/665173_Hammer_not_locking_back.html
https://www.ar15.com/forums/t_3_66/699774_Double_feed_problems.html&page=1&anc=bottom#bottom

Only time I call a disco worn out, if if the sear edge has cracked or chipped.  Short of this, I just time (or retime) the disco to the FCG to acheive the .001" ~ .003" needed free gap instead.

Yep, since the problem lies in the 9mm ramp bolt, and some of the hammer's top of FP Contact pads are not high enough to allow the ramped 9mm bolt to fully cock the hammer all the way back to the disco to catch the hammer rear sear.   To make the problem worse, you may luck out with hotter ammo that may sling shot the hammer back far enough at bolt opening to get the hammer back far enough for the disco to grab it, then with weaker ammo, bolt is not sling shot  back as hard during it blow back bolt action to get the hammer far enough back for the disco to retain it then instead.

So general rule on a 9mm non ramped bolt, you use the Colt 9mm hammer.


On a ramped bolt, you use the Modified style m16 type hammer isntead.


And again, notice the difference is heights of the top of hammer at the firing pin contact pad.

One last thing, and since the part of the none ramped carrier and the hammer that seats against it at primer ignition is also a part of what is holding the bolt forward for a brief second,  if you are going to use a 9mm ramped carrier, you have to either add hammer  spring tension, recoil spring or buffer mass tension for the tension you just lost by ramping the bolt.

As stated, the blow back action loves to snap FCG pins, and with the carrier ramped (loss the added tension of the hammer against the flat that the bolt take a little extra effort to cock back), the hammer end up being sling shot harder, and the faster you snap pins isntead.


To bottom line it, the reason that military did not pick up the 9mm blow back action, it's brutal on FCG pins.  
So instead of going to a blow back system, they went to a roller lock up system on the HK instead.